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o ^ 0F cc Vv'f NBS TECHNICAL NOTE 670

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

An Infrared Spectrometer

Utilizing A Spin Flip Raman Laser,

IR Frequency Synthesis Techniques,

and CO2 Laser Frequency Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (I) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute tor Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of the Office of Measurement Services, the Office of Radiation Measurement and the following Center and divisions:

Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center

for Radiation Research: Nuclear Sciences; Applied Radiation — Laboratory Astrophysics L — Cryogenics " — Electromagnetics L — Time and Frequency \

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1 Headquarters and Laboralorics at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington. D.C. 20234.

- Located at Boulder. Colorado 80302.
An Infrared Spectrometer

Utilizing A Spin Flip Raman Laser,

IR Frequency Synthesis Techniques,

and CO2 Laser Frequency Standards

e

'7echn,cc^J note, no. d>70,

J.S. Wells and F.R. Petersen, Laser Metrology Section

Time and Frequency Division

National Bureau of Standards

Boulder, Colorado 80302

G.E. Streit , P.D. Goldan and CM. Sadowski

Aeronomy Laboratory

National Oceanic and Atmospheric Administration

Boulder, Colorado 80302

-7

OP STAJTOASDe

APR 2 1976 no-t qzc,

. a 5"7-53

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

1 11 Issued January 1976
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 670

Nat. Bur. Stand. (U.S.), Tech Note 670 : 54 pages (January 1976)

CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office , Washington, DC. 2040

(Order by SD Catalog No. C13.46:670) $1.60
CONTENTS

pAQE

1. INTRODUCTION 1

2. SPIN FLIP RAMAN LASER CHARACTERISTICS 5

3. CARBON MONOXIDE PUMP LASER \k

h. INFRARED FREQUENCY SYNTHESIS TECHNIQUES 22

5. OPTO-ACOUSTIC DETECTOR 31

6. ENVIRONMENTAL PROBLEMS 3k

7. METROLOGY PROBLEMS 40

8. REFERENCES 48

I i i
LIST OF FIGURES

PAGE

Figure 1 3

Figure 2 6

Figure 3 12

Figure k 16

Figure 5 17

Figure 6 20

Figure 7 23

Figure 8 27

Figure 9 32

Figure 10 35

Figure 11 37

Figure 12 38

Figure 13 39

Figure ]k ^1

Figure 15 ^3

Figure 16 ^

Figure 17 ^

; 6

i v
An Infrared Spectrometer Utilizing a Spin Flip Raman Laser,

IR Frequency Synthesis Techniques, and CO Laser Frequency Standards

J. S. Wei Is, F. R. Petersen,

G. ,E. Streit, P. D. Goldan and C. M. Sadowski

The central part of this spectrometer is a cw spin flip Raman laser which

operates between 1900 and 1 800 cm . The spin flip laser has so far demonstrated

capabilities of resolving spectra separated by 0.01 cm and shows promise of

exceeding this resolution capability. When used with an opto-acoust i c detector

or with a detector and a flow system, it is capable of making a variety of measure- ments of environmental interest. Many of these environmental applications require

only wavelength metrology.

By using CO laser standards as frequency references and incorporating

infrared frequency synthesis techniques, spectroscopy with the spin flip Raman

laser can be put on a frequency metrology basis. A CO laser has been used with

a meta 1 -oxi de-meta 1 diode to synthesize a known frequency reference which was

used to stabilize the CO pump laser. The MOM diode also has the potential

for measuring the frequency difference between the SFRL and the CO pump laser,

a step which will complete absolute frequency measurements with the SFRL. The

progress toward achieving this goal, the potential capabilities of this unique

spectrometer, and some future applications are discussed.

Key words: Air quality measurements; improved resolution spectroscopy;

infrared spectrometer; infrared frequency measurements; spin flip laser;

tunabl e I R 1 aser

1. INTRODUCTION

As a result of continuing technological development in the laser field,

tunable laser sources are now available and many new possibilities for high

resolution infrared spectroscopy now exist. These tunable sources include the

dye laser and parametric amplifiers in the visible, and diode lasers, spin

flip Raman lasers (SFRL), and wave guide lasers in the infrared. The spectral

brightness of these sources has precipitated a revolution in spectroscopy.

We have constructed a cw spin flip laser which operates between 1900 and

l800 cm . This spin flip laser has, so far, demonstrated capabilities of

2 -1 resolving spectra separated by 0.01 cm and shows promise of exceeding this
resolution. When spin flip laser technology is combined with the infrared

3 frequency synthesis technology developed at NBS, one has the potential for a

unique infrared spectrometer. This spectrometer is being developed in the following manner. Because of the absolute frequency measurement and stabilization

work with CO lasers, excellent secondary standards in the intermediate IR

spectral region now exist. One CO device has been used with a metal-oxidemetal diode to synthesize a frequency reference with which to stabilize the CO

pump for the spin flip Raman laser and thus provide the requisite accuracy

and long term stability required for high resolution spectroscopy and chemical

kinetic studies. The diode has also demonstrated a potential for measuring the

frequency difference between the spin flip laser and its pump, a step which will

permit absolute frequency measurements. High resolution spectroscopy and

absolute frequency measurements are extremely useful for characterizing potential

lasers and molecular species of high scientific and technological interest.

Our group also has a novel NOAA developed opto-acoust ic (OA) detector which

is capable of measuring infrared absorption in gases at relative concentrations

r well below a part per million. This detector is very useful for doing spectroscopy on atomospheric pollutants and could be advantageous for investigations

of compounds of low volatility in technological applications.

The capabilities with laser stabilization, infrared frequency synthesis,

and high resolution spectroscopy, along with the spin-Hip laser and the new

detector provide us with unique opportunities to do some rather diverse experiments. The spin flip laser, stabilized lasers and opto-acoust i c cell are combined into the spectroscopic system shown in Figure 1.
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Some potential applications using this system are:

Category I (Environmental Problems)

1. Point sampling of atmospheric pollutants,

(laboratory spectra on collected samples)

2. Evaluation of the OA detectors for field use.

3. Measurement of chemical kinetic rates.

Category II (Metrology Problems)

1. Establishment of bench mark freguencies for laser diode users,

(moderate resolution)

2. Measurement freguencies in molecules involved in saturated

absorption experiments (high resolution)

3- Measurement of freguencies in molecules with potential for nonlinear lasers (e.g. molecules where the energy level of interest

is the sum of two CO laser freguencies, i.e. NH , S i F , , CH F, etc.)

h. Updating conventional molecular spectroscopy on molecules of high

sc ient i f ic interest .

These applications will be discussed in some detail after the following description of the spectrometer and its associated technology.
2. SPIN FLIP RAMAN LASER CHARACTERISTICS

7 8 9 Since many excellent references exist on the spin flip laser, ' only

a rudimentary sketch of the theory will be given. Our objective is to indicate

how the experimental variables at one's disposal (magnetic field, pump frequency?

input power, carrier concentration, temperature control technique, and InSb

resonator size) are related to the performance of the spin flip Raman laser

10 11 (SFRL) . We also point out some limitations of the laser ' that are generally

not widely publicized, as well as advantages for certain applications.

1 2 A useful model in describing the Raman scattering is indicated in Fig.

2. The magnetic field splits the Landau levels into electron spin down and

spin up levels as shown. One of the requirements for spin flip lasing is that

the magnetic field be strong enough to lift the upper spin level (spin down)

above the Fermi level. The magnetic field at which this occurs is called the

quantum limit. The diagram on the left indicates a field below the quantum

1 imi t

.

With the upper level above the Fermi level as shown on right hand side,

one can view the Raman scattering process as one in which the pump photon of

energy hv is absorbed, exciting an electron from the valence band to the

empty upper level, leaving a hole in the valence band. This hole is filled by

an electron from the lower Landau level (spin up) which drops down to fill

the hole along with the emission of a photon of energy hv which is shifted

from the pump photon by g (3H . This whole process (non-zero matrix elements

of the dipole operator between the conduction band, with its pure spin up or

spin down states, and the valence band) is made possible by the fact that the

valence band state is a linear combination of spin up and spin down states.

This mixed spin state is due to the spin-orbit interaction. At the end of the process
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there is one more electron in the upper level and one less in the lower level;

hence the net effect of the process is to flip the spin of one electron in the

conduction band. The more general process involves virtual transitions and the

exact match between photons and energy levels suggested in Fig. 2 is not required

The spontaneous output frequency, v , is given by

. v = v ± g»BH

s P

where v is the pump frequency, g the effective g-factor, 3 is the Bohr

magneton, and H the magnetic field intensity. Using an effective g-factor

of hS, one gets an approximate value of 23 cm per Tesla for a tuning rate.

In practice, the fact that the InSb is a resonator cor.pl icates the tuning

rate. Modes in the resonator have frequencies, v D , given by

K

V

R = 2&7 m

where c is the speed of light, L is the resonator length, n is the index of

refraction, and m is an axial mode number . For a 7mm long InSb cavity, this

mode spacing is about 5-35 GHz or 0.0075 Tesla.

Because of the existence of these cavity modes the output frequency of the

spin flip laser is not the same continuous function of magnetic field as is

the spontaneous frequency indicated by v . As the magnetic field is swept,

the output frequency may change at a rate less than predicted for v ,

then hop in a discontinuous manner to the next higher frequency mode.

These features are referred to as mode pulling and mode hopping. ^ They result

from competing processes due to the discrete modes in the indium antimonide

resonator.
13 The output frequency is given by the expression

Vs * Vr

V =

r + r n

s R

where Y is the resonator linewidth and Y is the spontaneous linewidth

K s for the Raman scattering process. (Y is approximately the lasing bandwidth

for the spin flip laser). Typical values are 0.15 cm for Y and 0.0*; cm

K

1 5 -3

for T for electron concentrations on the order of 8 r. 10 cm

s

. 14

In practice, one can operate in the so called spin-saturation regime

(where the power is limited by the relaxation rate from the spin down to spin

up level) with several closely spaced off-axis modes operating simultaneously

and this mode hopping behavior is not discernible. The resulting linewidth

(the order of 0.01 cm ) is still often less than pressure broadened lines of

gases of interest in certain applications, as is shown later.

The coarse tuning is accomplished by pumping the spin flip laser with

different CO laser transitions. This method turns out to be preferable in

some instances to large tuning with a super conducting magnet for reasons

appearing in the next paragraph. The preceeding discussion indicates the

obvious frequency dependence on the pump frequency and magnetic field. A

more subtle magnetic field dependence relating to the intensity appears in

the following discussion.

In order to operate at frequencies far from the energy gap, i.e., about

-1 . . . ]h

50 to 200 cm away, it is necessary to consider the lasing condition,
R exp (G-a) lU 1

where G and a are the gain and loss per unit length, L is the resonator length

and R is the sample reflectance. Of particular importance in this expression

i s the ga i n :

G = hv n

P

N SI

e o

where T is the intensity at the crystal which depends on the pump power available

and the f number of the input lens. The resonance enhancement factor, S, is

given by:

S = A

where hv (H) = E + (£ + i) h co (H) + i g<''(3H. Those quantities not previously defined include, I, the orbital quantum number, A, a constant of the

- 9 3 order of 2.3 x 10 , and N the excess (spin up over spin down) electron

e concentration. The product h v (H) is the energy difference between the

top of the valence band and the bottom of the upper spin state of the particular Landau level (and hence depends on magnetic field) involved in the

scattering process. The dependence of v on H may (depending on sample

concentration) dictate keeping H small and accomplishing the gross tuning

by selecting a pump frequency v closer to the desired SF frequency.

The resonance factor presents some difficulties as one moves away from

3 the energy gap. Table 3 indicates the relative value of S/v at different

P pump frequencies. The most effective method of compensating for the
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decreased gain at lower frequencies is to increase the pump intensity, however

some limitations exist here. One can also increase the carrier concentration,

however this is partially compensated by an increase in the spontaneous linewidth T . F and the Fermi level are related to the carrier concentration

s s

by the following proportionality.

. T a E r a (N ) 2/3 she

Since the Fermi level is also related to the carrier concentration, the

upper spin level moves through the Fermi level at different magnetic fields for

different concentrations. The Fermi level also is somewhat magnetic field

(as well as temperature) dependent. The primary practical results relate

to two considerations. One is the power output as a function of magnetic

field. This is best illustrated by Fig. 3 which shows some results by the

1

6

Heriot Watt group. The spin flip laser power output is plotted vs magnetic

field for two different electron concentrations. For some experiments,

2 it is desireable to have the most power possible at 0.2 Tesla, hence a 2.5

15 3 x 10 /cm sample is preferable. A second consideration is to obtain the

narrowest possible linewidth. In this case it is advantageous to choose a

low concentration sample where V and the gain bandwidth are smaller. In

this case, it would also be preferable to select the spectral feature of

interest to be close to the InSb band gap.

Several different techniques exist for cooling the InSb resonator to

the desired 2K to 20K region. One technique is to use a commercial cryogenic

refrigerator if one's budget permits this initial investiment. The other

two methods involve the use of liquid helium, either for immersion in the

11
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liquid, or cooling by a cold finger. Each technique has both advantages and

drawbacks. We have used both techniques in our operation.

For immersion, we have found ZnS windows sealed with a suitable low

temperature epoxy to give fairly reliable operation, although a vacuum failure

may occur upon repeated recycling. Pumping on the helium to get below the

lambda point is mandatory in order to avoid scattering of the infrared radiation

by bubbles in the helium. The dewar requires over night precooling with liquid

nitrogen before helium transfer if helium consumption is to be minimized. The

additional set of windows in contact with the liquid helium can present some

interference problems, particularly if the window coatings are not good. The

major advantage of this technique is that one can be reasonably certain of the

InSb surface temperature.

Cold finger operation is simpler once the initial difficulties are overcome. The cold finger in our apparatus is constructed of oxygen-free highconductivity copper with about 2~3 mm thickness separating the liquid helium

from the InSb resonator. Vapor deposition of gold on one surface of the InSb

appears to improve the indium solder bond between the sample and the cold

finger. Our procedure is to precool the helium reservoir with liquid nitrogen

then syphon this liquid to the nitrogen reservoir of the double dewar. This

procedure reduces lead time for helium transfer to about 30 minutes. Cold

finger operation is the obvious solution for operation with the resonator

external to the InSb crystal. However, several undesireable effects may be

present in this configuration. For instance, the output power may be less

stable than in the immersed case. Also, if the input beam is chopped, the

crystal can change temperature by larger amounts which can give rise to a

frequency modulation, or chirping. To date, however, we have not observed

these effects.

13
3. CARBON MONOXIDE PUMP LASER

An ideal pump laser for the spin flip laser would be one whose lines

are separated by about 50 GHz, with several watts of power available on

each line. Unfortunately such a laser has yet to be developed. The next

1 ? 1 6

best laser is perhaps the CO laser. The energy values for the C laser

may be obtained from the following term value expression 18

T(v,J) = go (v+i)-co X (v+i)

2 + co Y (v+i) 3 e e e e

CO Z {v+±) k + to a (v+]) 5

- uo b (v+i)

6 +

e e e e e e

J(J + 1) B - a (v+Jr) + Y (v+i)

2

- 6 (vH) 3 + ]

J

2 (J+1)

2 D - 6 ( e e v+i) + lMv+4) 2

- '"J

+ J

3 (J+1) 3 h - n (v+;) +

••• e e

-1 where the constants have the values, given in cm as indicated bel ow:

co X

e e

co Y e e

CO z

e e

co a

e e

oj b e e

co e = 2169.813 5802(881)

13.288 3076(^35)

1 .05 1127(758) X 10~

5.7^0(541) X 10'

9.831 (162) X 10'

3- 1660(173) X 10"

7

B = 1 .931 280 8724(^3)

a 1 -750 Ml 21 (729) X 10

-?

Y

e = 5.^87 00(186) X 10

6 = 2.5^1 (156) X 10

-8

7

-6

D - 6.121 468(291) X 10

e

)k
3 = 1.526(199) X 10

-9

n = 1.8050(15*0 X io" e

H = 5.8272(597) X

10' e

n = 1-7375(229) X

10"

10

12

13

The laser frequencies are the term differences

T (v + 1 , J + 1) - T ( v,J)

where v is the vibrational quantum number and J is the rotational quantum

number. The resulting frequency differences between levels are indicated

in Fig. h which points up the frequency spacing problem.

Since carbon monoxide lasers are not readily available commercially, we

have constructed and used several lasers designed at NBS . We describe model II

which is currently in use. This laser operates over a 1900-1700 cm range

and was designed for high output power at 6 pm to compensate for the fall off in

resonance enhancement as one moves away from the band gap.

The resonator frame consists of 15 cm square aluminum end plates which

are epoxied to k quartz rods, each 2 m long by 2.5 cm in diameter. One end

plate holds a grating mount with an integral micrometer drive; the other end

plate supports a mirror holder driven by a PZT piezoelectric stack or transducer

Adjustable irises at each end are used to force the laser to oscillate on a TEM

mode. Power is coupled out of the cavity through a 7mR, 90% reflectivity

mirror. The second surface is flat and AR coated. The t ransm i

t

tance of the

nominally 90% reflectivity mirror is shown in Fig. 6. The transducer can move

the mirror about 10 urn. The sweepable drive voltage for the transducer comes

from a standard NBS unit which was developed for laser stabilization. The

drive unit has provisions for dithering, ramping, and servo correction. The

other end of the resonator is terminated by an adjustable grating. We have

oon

15
ioc

in QJ

— U) ft fl) Pt T|

C 3 3 zr -s X)

O -i _. Cl in O cu "O fD

rl- r-t

g 3 CD CL JD

Ol T3 Q) in 0)

—C

cr- 3 r-t

— -1 -h CD

O 3" r-t in -n 3

CD ,—^ CD 0>

— (DO

S3 - rt

O (I) "I

—

-ti vn 3 — fD CD O -— £ 3 in <-+ 3 in

-s

— cu zr

n

r-t 3 £ CD CD -h

"D zr -<

— -i inO

C T) r+ "5 3 O O O zr r+ —|

T> -1 c -h o o

-1 r+ -^

O 3 r-t r-t ZT r-t <

ID TO

T (t

T—

— -l fD fD + QJ

r-t in qj in c_ in 3" n- fD — —' CD

CD -i

— < fD - n

0) 3 r-t QJ -h

in Qj ID -s

— r-t c_ cr -a in QJ C fD O -• 3 fD O + <-t 3 3 r-t in -i £ — OJ —• fD -h — r+ "O -5 fD i 3 in — "O — 3

_.

O

O fD IQ 1 "O r-t- a. 3 Qj zr

—

-•

w t rtH vJ

cu -ti

— o

cu

— -h QJ 3 O -—

O

in

_— -i IQ -h <

fD

O CD ^

-IO

C 0) rt 3

-+.—< cr zr <_ Qr-t fD O fD ~ r-t -| < • -*

zr r-t -< fD cr ^D

fD

O QJ

O

O rt 3 —I O

r-ta - T QJ

n

— o fD • m

n

qj in in 3 3 O fD -ti < ' in -j -i —1 — —

-.

— r-t fD zr cr

r-t 3 O XI fD -i _. _. c QJ O 3 r-r fD r+ r+

—1 3 QJ ZT 3 -1 — 3"

(/> r-t fDO QJ

O fD

fD < 3 3 in

l/i in in Qj T) zr cr Qj cr — —' O fD 3 0) r-t -h £ rt (D -1 — -Q 1 3 £ • O C 0)

fD -h 3 QJ -Q

Qj fD

n in 3 C

-1 3 fD -Z. r-t fD

fD JO

C QJ

C 3

-OC 3 -s 3 O

—> fD (D CD

—

S3 3 -S 3 CD ^ O — QJ cz if —» -< o — 3

i_D QJ

— cr

O ^ —. fD O

"U -! I

T| < I O QJ < CD -I _- in C "D

fD

O

in 3 oo

-Jo

-Cr' o

oMo

-J

CD' o

jom

H2 -\

COo : o o<

CO.

S3o|

II CO

COo 1

co - o : 1—1 L .

COoo

16
ooo

L.J oo , . <^J 1

L.J

-c^ u

>-u : z.

LLJ -. , . N. C;V^

ll: CrC u~ oo

CNooCN

COoo

-oO

^xr

A—"

-•-I

__;

h

-O-

•CNIS) ^z =ti

F^

—f ^

Tn

(%) 33NVillWSMVHi

>> M«m>

•—M

>» o o 03 c •—

o> <4-

ZJ • 0) cr E 1_

03 P.

!_ !_ H- CO <u • X 4-> un CO

(T> •—

-d -C

C i— o (U _c • 

4->

4-> CM .— o • S- c LA D !_

<4- c o

a> !_ 03 CD J_ S —

01

M E

CD 0) _Q 0) !_

(I) in Cn

(/) (U c

03 4-J • 

r— 03 01

s_ o

<u 0) o

X3 Q. -C •— O u Xo 5 >~ c o -Q o c E E — r-L c <D o t/> LT\

1_ i— VD

rao <U o

_c c c 1- o o

o>

1_ • -D o 03 i_ > o

1_ o 4-> .— -Q E 03

03

4-J r- TJ 3 5 C

o.

O 0)

4-1 -C 4-> 3 </> X • o a) c

in o

14- •— 0) • 

o -O cn x; 03

<u 4-J c !_ o Dl 03 c c U fc

ro a) =L

+j .

c

4-> 0)

O vO • 

> •— F 0J 4-J 03 Ul 5 03 JC c 1_ 4-> 03 -o 0) U c CL c

h- 03 o "

LA

a;

L.D

cn

17
attempted to make a compromise between large dispersion and having a reasonable

angle (the diffracted beam misses the rear output window for large angles) to

permit use of the smaller power output from the grating. The particular grating

in use has 2^0 groves/mm and a 26 h$ ] blaze angle. The relative efficiency

1 9 for S polarization falls from about 95% to 5 um to 90% at 6 ym for this model.

Table II shows the relative power output versus frequency for the CO lines

which typically oscillate in this laser.

The 10 mm I.D. discharge tube is 192 cm long with Brewster windows at

each end. Air spaces between the windows and resonator reflectors are sealed

with bellows to avoid air currents in the optical path. The discharge is

split with a single anode in the center and cathodes at each end to avoid undue

heating due to ion bombardment which would result from the single cathode

arrangement. All three electrodes are platinum "hollow cathode" elements.

A 30 mm O.D. concentric cooling jacket surrounds the discharge tube proper

and a h mm I D tube which connects the discharge to a ballast region also allows

communication from the anode to both cathodes. This connection is very beneficia

in "sealed off" systems; otherwise, the power gradually diminishes during the

course of the day due to impurity build up associated with cataphores i s

.

The connecting tube has stop cocks between the anode and each cathode to

prevent the discharge from firing through the connection tube at start up. The

discharge tube is shown in Fig. 5-

Two different cooling schemes are used. In both cases methy 1 -eye 1 ohexane

is used as the coolant. This coolant does not exhibit the viscosity of ethyl

alcohol at reduced temperatures and does not freeze unt i 1 lh7 K. In one scheme,

the methy 1 cycl ohexane is circulated through a heat exchange coil immersed in a

mixture of dry ice and alcohol. A temperature of 203K can be maintained with
Multiline power output vs frequency

Grat i ng

read i ng

936.5

926.5

918.5

907-5

898

878

868.5

858.5

848

835

828.5

808.5

798

788

777

763-5

757

744.5

736

724.5

714

709-5

692

680.5

669

636

625

615

600

579

568.5

556.5

545

530

520

508

496

480

469

^56.5

445

421

404

mi crometer

in 0.001 cm

Table I l

y for CO pump laser.

Operat ing frequency Power relative

reg ion i n cm"

1

to maximum output

1915 .24

1911 .28

1906 • 45

1902 • 32

1898 • 36

1894 • 55

1884 • 65

1880 .81

1876 .77

1872 .83

1867 .24

1863 77

1855 • 99

1851 1 .00

1847 76

1842 • 73

1837 .26

1834 • 73

1830 .26

1826 .44

1822 .86

1817 • 55

1813 72

1809 .75

1805 .46

1801 • 45

1788 53

1784 .60

1780 34

1776 36

1767 • 47

1763 .49

1759 • 43

1755 .44

1751 34

1747 -19

1743 • 34

1738 37

1733 .10

1729 .24

1726 .28

1722 .26

1714 .18

1709 .14

19
LQC

ON

— rt oo 3 zr 7T

fD fD

Qj rt rt rt n C 3"

rt rr

3" fD

O

fD -h

CT 1/1 o O o rt l/l rt -< a O 3 c 3 3 3

fD

a

O rt -h -«

—« —•

0) rt O (/i zr fD

fD QJ ~l cr n o Q. O c —• O rt l/l n —• rt 3"

=3 zr 0) in n> in i— Oj KV Qj 3 o o ,-t s: 7T Q_ c TOCD fD CD cr

rt a> o • TJ - —i

o_

CD fli

—• 3 l/l on

fD

— * o 3 • Id m 3 —1

ft) > CT X i

> n

fD

O o o < Tl

rt fD

"1

m

-h 0)

m

O

—•

<—>

~1 -^ —1

O "a

33 • —I

nO

rt

Oo 3 —1

C

m 3 rn o CT —i

fD s: rs

QJ 3> CO 3 — I— I

rt I—

m fD3 —.

UD 00

—« 3 rt O ro

fD 3" 3 • 3 -J3

rt

— • l— 3

l/l

O s z o

3" > CI CT — • UD O Ni

3"

n

n 3 o3 QJ fD 3

l/l C_ o

oom

50o

oon

3:3>

33CI o

20
this arrangement. At laser operation near 6 ym, however, lower temperatures

are required and a second cooling coil in series with the first is immersed in

a methylcyclohexane bath. Liquid nitrogen flowing through an additional coil

in this bath cools the methylcyclohexane to near its freezing point. With

this system, a temperature of 158°K can be attained. This second stage is

bypassed for 5- 3~5 - 5 ym operation.

20

The gas fill mixture in the CO laser is a premix of 6.3% Xenon,

6.3% nitrogen, G.?,% carbon monoxide and the balance is helium. Typical fill

pressures vary between 2,000 and 2,700 Pa (15 and 20 Torr), depending on the

intended operating temperature. The discharge current is typically around 10mA.

Each side of discharge is powered by a 25 kv power supply with 200 k ohms of

ballast resistance in the circuit.

The transducer controlled mirror permits one to adjust the frequency of the

CO laser within its gain profile. A reference frequency is synthesized by a

method described in the next section. The CO laser frequency is locked

30 MHz away from this reference frequency by means of a 30 MHz discriminator,

an operational amplifier and filter, and the previously mentioned driver unit

which adds the amplified correction voltage to the bias voltage already on

the PZT transducer.

21
'*• INFRARED FREQUENCY SYNTHESIS TECHNIQUES

One of the prime objectives of the infrared frequency synthesis (IFS)

program at the National Bureau of Standards (NBS) , was to determine the

frequency of the 3-39 um P(7) transition in methane. In the process of

achieving this goal, two definitive CO laser frequency measurements were

3 made. Using these two frequencies as a basis, a thorough program to determine values for stabilized CO laser frequencies was undertaken and completed

at NBS. By using two CO lasers, a klystron, and an appropriate diode one

21 may synthesize any frequency between and 100 THz. A list of two-laser,

CO comb i nat ions for generating frequencies in this fashion has been compiled.

More recently, the IFS technique has been used to measure the frequency of

22

the Xenon laser at 2.0 um. A long term objective has been to extend the

technique to the visible. A secondary objective is to laterally extend the

technique to use the CO laser synthesizer with tunable lasers and make

absolute frequency measurements.

An example of two-laser infrared frequency synthesis is shown in Fig. 7-

To accurately measure the frequency of a laser, v a frequency, v , which is

close to the frequency to be measured, must be synthesized. The difference

between these two frequencies is an intermediate frequency, ^.p. typically

less than one GHz. The unknown laser frequency is:

V

M

= V

S

± V

IF

v

s = £v1 + mv2 + nvyw

.

V. and v are basis laser frequencies which have been determined by prior

synthesis measurements, and v .. is a microwave frequency. The quantities

uW

22
> CO b > «* + 1

CXI

*3- s q: =1 LU

cc h^l y— ^

(_> _J LU <=C Q- 2:

<= o

CT» CO c

<D j_ > O • — <+- cn -0

QJ QJ

s_ cn

fD

n

cn QJ

QJ J3 u >- c fD

QJ E D

cr s_

QJ

!_ •—

M- U- .

Q. < E

fD

Li_ cn —

cn QJ , QJ -C

O

J=M i_

+->

M c E c > O

cn (J > s_ U 

O c fD

QJC ~ 3 cn xi

O" •— c

QJ cn fD

V_

<4- QJ 4-) _C c

TJ \~ QJ

QJ E

!_ 0)

ro !_

!_

D

<4-

N cn c nr fD •— h- QJ£

14-O

N >-

-l-J

O

QJ L. c 

QJ QJ CL JZ 3 E fD CT

fD 1_ QJ X QJ L- UJ 4-> -1-

23
I, m, and n are harmonic numbers which are allowed both positive and negative

values. The quantity (l + \t\ + |m| + |n|) is called the mixing order. 2 ^

The harmonic generation as well as the mixing which produces the intermediate frequency to be measured, occurs in a suitable diode, typically a 2k 25 tungsten catwhisker on a nickel base. ' J Two relevant points are indicated

in Fig. 7- First, the laser beams are polarized with the electric field

vector in a plane which contains the catwhisker antenna. Second, the

angles between the laser beams and antenna are given by long-wire antenna

26

theory. According to this theory, the angle between the antenna direction

and the direction of the first maximum in the radiation pattern is given by

, -1

) = cos max (,-MZIi)

where A is the wavelength of the radiation being coupled to the antenna and L

is the distance between the tip of the tungsten wire and some discontinuity

in the wire, either in shape or direction. Although some contention exists

as to the type of coupling that prevails at 3«39 l inn (perhaps a conical antenna)

our experiments indicate that the long wire antenna theory is applicable at

5.3 I'm. Our measured value of =11 ±1 is commensurate with the antenna

m length at 100 vim. The unetched diameter of the antennas used in this work

is 25 ym.

The physical mechanism of the harmonic generation in these diodes is

not universally agreed upon. Theories postulating asymmetric tunnelling due

to geometry, quantum mechanical scattering, field emission, and barrier

tunnelling have been made. In some of the tunnelling theories voltage is

considered the independent variable; in others the current appears as the

wave fundamental quantity. Since the antenna coupling tends to be associated

2k
14

with generation of currents and the existence of currents near 10 Hz

has been demonstrated, this latter theory has considerable appeal.

27

The essential ideas of one tunnelling theory are as follows.

The diode consists of two dissimilar metals (nickel and tungsten in our

case) separated by some unspecified insulating barrier, probably an oxide

which is several lattice parameters thick. If 0.

| and

2 are the work

functions for metals one and two and V is the voltage between the two

metals, the expressions for the electron tunnelling currents given by

28

S i mmons a re

:

Ae

f

' 2 2-nht

2 e T0 : vn ' T(0 9 + eV)

(0 + eV)e

for V > and

, = "Ae

r 2uht

v

-TM, eV) e"

T( "0, - eV) ?

for V<0

The quantity A is the contact area of the diode, which may correspond

o to a 500 - 100 A radius of the tip, and t is the approximate barrier thickness,

o

15 - 20 A. T is given by

T

4ir(t - t ) J 2m 4-TTt -J 2m

h h

where t- and t

1 are classical turning points in the barrier penetration

problem. The quantities e, m and h are the electron charge, electron mass

and Planck's constant, respectively. 17 h i 1 e no further use of the above expressions

is made here, they do show one explicit current-voltage relation and indicate some

dependence of the current on the- physical dimensions of the junction of the diode.

25
It is then assumed that the current, I, through the diode consists of an

external bias, i , plus currents which the laser radiation induces in the b

antennas. For example,

I = i, + i coso3. t + i_ cosu)„t

b 1 12 2

This current and the tunnelling current expressions are combined to give

the voltage as a function of current

v = g(D

where g(l) is a non-linear function of the total current.

The harmonic generation is mathamat i cal 1 y evident when g(l) is expanded

in a Taylor series about the bias current, i, .

dij

: g(i

b

} + dT ).

(i

i cos V + ! 2 cos

o),t

)

'b

2

1 d

2 q \ /. . x 2 +

y, —|- 1 d 1 cosu^t + i 2 coso^t) + ...

'b

The general term corresponding to the nth harmonic has not been shown

since we can illustrate our immediate use with the second order term. For

stabilization of the CO laser, we have not found it necessary to use an

external biasing current on the diode; hence i equals zero. An examination

2 2 the second order term indicates constant terms proportional to i .. and ias well as terms involving sum, difference, and doubled frequencies. These

constant terms represent rectified optical signals and can be used to

advantage to monitor coupling of a particular radiation to the diode.

This is accomplished by chopping the laser radiation and adjusting the

optics for a maximum value of the modulated rectified voltages on an

osci 1 1 iscope, (typically indicated in Fig. 8), which shows rectified

signals and the resulting beat notes.

26
!

I

i

!

. i-

- — — - — - j

: \

'

!

i -

^4i'J h^-imiiii'iihi , -'-|fa^

!

! !.: j

:

:

f. : . . , ,ri-r-i-K-

- •

-h-h-

- ! ' f

! M f II'' 4- . i

:

i fKi;-! L L J**?, .. .

i ,

Figure 8, Rectified voltages for CO and CO lasers on a metal oxide

metal diode, a) Rectified CO induced voltage on a 5 mV

per division scale. b) Rectified CO induced signal on a

5 mV scale. c) Beat note at 30 MHz when a k(> GHz signal

from a klystron additionally irradiated the diode. Signal

to noise ratio for the beat note is 30 db and dispersion .s

1 MHz/division. The spectrum analyzer bandwidth is 100 kHz

and sweep time is 1 ms per division.

27
The coefficients of the frequency sum and difference terms are proportional to the product of the two laser induced current amplitudes. Thus,

by compensating with increased power from one laser, mixing experiments can

be done even though the other laser induced signal may be weak. Significant

non-linearities, such as burning-up of the tungsten antenna, will ultimately

limit the extrapolation of this technique. Hopefully, power compensation

by one of the synthesis lasers may help solve the problem of measuring the

frequency of the relatively low power, spin-flip laser. We defer that

discussion until the latter part of this section.

In order to see the des i reab i 1 i ty for I FS techniques with the stabilized

CO lasers in our spectrometer, we need only consider some of the properties

of CO and CO lasers.

The CO laser is unique in that it has over 100 lines (more or less

uniformly spaced) extending from 9 to 11 urn, all of which can be stabilized

to standing-wave saturation resonances observed in the 4.3 urn fluorescent

radiation. The fractional frequency variation of lasers stabilized in this

k -11 -i -2

way at the National Bureau of Standards is 3 x 10 t for 10 £ t > 10s.

It is estimated that the fractional uncertainty in the resetability of each

CO laser is about 2 x 10 . Further, the absolute frequencies of these

9 lines are known to within about one part in 10 .

The lines in the CO laser do not have the same desirable separation as

in CO ; in fact many transitions nearly overlap, and unfortunately the CO

stabilization technique cannot be used for CO. We have used an alternative

stabilization procedure for CO, as shown in Fig. 1. Here the CO laser is

locked to a frequency which has been synthesized from the second harmonic

of a stabilized C0„ laser and an appropriate microwave frequency. Both the

2nd harmonic generation and the mixing are done simultaneously in the point

28
contact diode with the properties indicated in the caption of Fig. 8. Over

100 CO lines lie within ^f0 GHz of the second harmonic of some C0„ laser line

and may be stabilized in this manner. The CO laser would then have nearly

the same long term stability as CO , and its absolute frequency would be known

9

to within a part in 10 . This procedure increases the utility of the CO

laser as a pump for a tunable Raman spin flip laser to be used for high

resolution spectroscopy. Also, as a monitor of the CO laser operation, one

can observe the output from the diode and make appropriate adjustments to

insure that the CO laser is operating in a single mode and on a single transition.

The problem of measuring the difference frequency between the pump laser

and the SFRL output is complicated by three factors. First, the spin flip

and pump signals have mutually orthogonal polarizations, which originally

presented difficulties in coupling to the diode antenna. Second, the power

output from the SFRL is low compared to levels generally used for synthesis

in MOM diodes. Third, the large col i near 1 y-t ransmi t ted pump power makes

focusing the weak SFRL output beam on the diode difficult.

Long wire antenna theory indicates that for best coupling the antenna

should be rotated in the plane of polarization by the angle with respect

to the beam direction. normally, our diode holders have provision for

rotation in the horizontal plane only. A tapered fixture which tips the

diode down 11 in the vertical plane while maintaining an 11 projection

in the horizontal plane with respect to the beam direction has permitted

5 um signals with either polarization to be coupled to the diode.

15 "3

An InSb crystal with an electron concentration around k x 10 cm

has a maximum in the power output curve at about 0.2 Tesla as indicated in

Fig. 3- We have used a crystal with a carrier concentration of 2.5 x 10

29
-3 cm and obtained an estimated 30 mw maximum output power near 0.205 Tesla.

This point of operation is selected since the frequency difference between

the CO pump and the SFRL is predicted to be about 150 GHz corresponding to

the 2nd harmonic of an available klystron. Since the power available from

the SFRL is low, it is deemed desirable to keep the mixing order as low

as possible in the initial experiments. The SFRL power density is increased

at the diode junction by focusing it down with a 2.5 cm focal length lens.

Sufficient transmitted pump power (although the radii of curvature of the

pump and SFRL signals are not generally equal) might also be coupled to the diode

through the same lens to produce a beat note between the laser and the

75 GHz klystron. We are exploring the possibility of measuring the frequency

difference in this manner.

While the MOM diode has considerable potential for heterodyning the

SFRL signal, a reliable procedure needs to be developed for coupling the

laser signal to the diode. Some of this development involves improvements

in the SFRL itself. An external resonator configuration would permit

single mode operation of the SFRL, a single wave front for coupling to the

diode, and hopefully greater power output. The inclusion of a Brewster angle

window within the resonator could also reflect some of the troublesome

transmitted pump radiation from the beam. An additional improvement would

be to bring the pump and spin flip beam to the diode along separate path

lengths as proposed in Fig. 17, for example. Separate focusing would afford

some control over the relative powers, permit parameters to be evaluated

on a quantative basis, and pave the way for routine heterodyning procedures.

30
5- OPTO-ACOUSTIC DETECTOR

The development of air pollution monitoring equipment capable of detecting

the presence of atmospheric trace constituents at concentrations as low as one

part per billion has led to a number of highly sensitive measurement techniques. One such device developed on our laboratory and elsewhere is the

resonant opto-acoust ic detector.

In this device a confined gas is irradiated by a laser tuned to a vibrational frequency of the molecules in the gas, and consequently, some of the gas

molecules reach an excited state. Subsequent collisional de-excitation of

these molecules results in a thermal i zat ion of the absorbed energy and therefore a pressure rise in the gas. Thus, modulation of the intensity of the

irradiating laser produces pressure fluctuations which may be detected by a

suitably mounted microphone. If, in addition, the laser is modulated at a

frequency which corresponds to one of the natural acoustic resonances of the

gas confining cavity, the acoustic signal generated will be enhanced by an

amount proportional to the cavity Q, thus, nreatly increasing the system

sensitivity. The basic features of such a system are illustrated schematically

in Fig. 9 (details available elsewhere) which shows a cylindrical sample

cavity with a microphone mounted flush with the cylindrical wall. In addition,

Fig. 9 indicates that we have greatly enhanced the chance for absorption of

the irradiating laser beam by multiply reflecting it between two mirrors

forming the end walls of the cavity, thus further increasing the system

detection sensitivity.

Measurements with this system have demonstrated acoustic Q's exceeding

750 and detection linearity for all absorber gas concentrations below a

saturation threshold which corresponds roughly to an absorption coefficient

-2 -

1

of 6 x 10 cm . Since the opto-acoust i c detection sensitivity is directly

proportional to the absorbed infrared energy, direct comparison with conven31
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32
tional measurement techniques which depend only upon fractional absorption is

somewhat difficult. Measurements to date indicate, however, that a signal to

noise ratio of 1 is achieved at a bandwidth of 1 Hz, with a 1 watt irradiating

laser and a gas fill having an absorption coefficient of 3 x 10~' cm"

1 at

53,000 Pa (400 Torr). To achieve comparable sensitivity, a conventional absorption

detection system would require an effective absorption path length of -100 meters

compared to the 20 cm opto-acoust i c absorption cell.

The opto-acoust i c system thus currently displays a linear detection

range exceeding 7 orders of magnitude. The system noise is currently dominated

by electronic amplifier noise whereas the fundamental limitation is the

thermal acoustic noise on the microphone element itself. Improvements in

microphone and electronic design should allow us to reach this fundamental

limitation. Improvements in the cavity optical design should also result in

greater system ultimate sensitivity.

Since the detection system requires the conversion from vibrational

excitation to random thermal energy via a gas kinetic collision mechanism,

some loss in system sensitivity is bound to occur as the cell pressure and

thus the molecular collision frequency is reduced. We have, however, made

relatively low level absorbtion measurements at pressures as low as 700 Pa (5 Torr)

indicating that measurements even at low pressures may be made by the optoacoustic technique. More work in this area is needed to see if specialized

microphone design could improve the low pressure measurement capabilities.

A device such as this offers advantages on two counts. One, materials having

low vapor pressure at room temperatures could be investigated and two, particulate scattering (which could be a problem in transmission spectroscopy)

does not adversely affect operation. A spectrum obtained by using the

opto-acoust ic detector and spin flip laser is shown in the next section.

33
6. ENVIRONMENTAL PROBLEMS

The spin flip laser (including the CO pump), opto-acoust ic detector, a

stabilized CO laser, and some additional infrared frequency synthesis apparatus

are assembled into the system shown in Fig. 10.

A stabilized CO. laser, microwave source, and point contact diode are

used to synthesize a suitable reference to frequency offset lock the CO laser.

For example, P ( 1 7) in the 8 to 7 band for the CO laser is stabilized by

operating the CO laser on P ( 1 8 ) in the 10.6 ym band and phase locking the

microwave oscillator to 46.608 GHz. When these three oscillators simultaneously

irradiate the diode, a 30 MHz beat note is produced which is used to stabilize

the CO laser frequency. The correction voltage generated by a conventional

discriminator is amplified and applied to a piezoelectric transducer on the

CO laser. Stabilization of other CO laser lines requires additional klystrons.

A reference oscillator and power amplifier control the speed of the

synchronous motor which drives the chopper. The chopping at the optoacoustic detector frequency is done preceeding the InSb sample to minimize

heating of the crystal in the cold finger configuration. The beam from

a light-emitting diode is also chopped, and this signal provides the reference

for the phase detector.

The signal from the opto-acoust i c detector is also fed to a phase detector,

and its output is fed to the Y-axis of an X-Y recorder. The X-axis is driven

by a signal derived from a Hall probe in the magnet.

In an alternate simple absorption experiment, a mirror is used to direct

the spin flip output through a 10-cm-long absorption cell equipped with

Brewster windows. The filter may be a monochromator , and the detector

currently in use is a gold-doped germanium diode.

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The application of the first scheme to detection of pollutants is

29 demonstrated by the nitric oxide spectrum in Fig. 11. Nitric oxide has

a large number of strong lines within the frequency capability of the

system. Tunability of the spin flip laser permits selection of a transition

which avoids the spectral lines of other pollutants. By tuning the laser to

the R(?) transition, a sensitivity of one part in 10 of NO can be achieved

with a 30 mW SF signal.

The operating pressures in the opto-acoust i c cell (about 6,700-53,300

Pa or 50-^00 Torr of buffer gas) so far have precluded h i gh- resol ut ion nonpressure-broadened spectroscopy. However the spin flip laser does have this

capability. This resolution is demonstrated in Fig. 12 where the alternate

system has been put into use.

The system is not limited to use for detection or identification of

pollutants which have a fundamental band overlaping the 5-3 urn region. For

example several molecules have overtone bands which also fall within this

region. As an example, the spectrum of SO is shown in Fig. 13- This

characteristic signature of S0_ in this region provides an interesting contrast

with the simple and well known spectra of NO in Fig. 11.

Finally we point out the potential for chemical kinetics investigations

in this region. Due to the great selectivity and high sensitivity for

molecular detection achieved with the tunable IR radiation of the SF laser,

this system should prove excellent for chemical kinetics studies. Once an

appropriate spectral feature has been chosen, it can be monitored quantitatively

throughout the reaction providing the frequency of the SF signal does not

change. This is assured by the frequency locking servo on the CO laser

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and the feedback control which maintains the magnetic field at a constant

va lue.

Either the opto-acous t ic detector or the simple absorption cell may be

used to monitor the gas concentration in a flow tube system.-^ The optoacoustic detector, however, unless modified to obtain low turbulence flow

characteristics, is suitable only for very low flow velocities and hence

measurement of slow reactions. Because of this constraint, a flow tube

system incorporating an absorption cell is currently under construction to

study kinetics of some of the free radicals detectable within the available

tuning range of the SF laser.

7- METROLOGY PROBLEMS

Most of the technological problems that the spectrometer could help

solve would involve making absolute frequency measurements. One category

would be to provide reference or bench mark frequencies that diode laser

users could find beneficial. Diode lasers at present do not have sufficient

power to use with meta 1 -ox i de-meta 1 diodes, and while they do afford excellent

31 potential for high resolution, they appa rent ly are not exactly reproducible

from day to day in regard to their frequency-current relations. Large

facilities where diodes are in use are forced to refer all measurements to

32 an internal standard whose absolute frequency is not known. Fig. 14 indicates

the type of spectrum that could be useful as a reference in regions where

the spin flip laser operates.

A second category of experiments is related to some current activities

33 regarding isotope separation and a related class of two-photon experiments

involving C0_ lasers. Since the C0„ laser has very desireable properties,

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such as high efficiency, high power, cleanly separated transition frequencies

and scaleabi 1 i ty , several proposals have been put forth to use CO lasers to

construct an optically-pumped NH laser involving two input CO photons and

to get 16 um laser action from the NH as indicated by the energy level

diagram in Fig. 15- Since the two photon total tunability is about 100 MHz,

the energy levels must be known to this accuracy. The 5-3 um operation of

the spin flip laser overlaps a large portion of the frequency region equal

to two CO laser photons.

Fig. 16 indicates the spectrum of NH_ obtained with our spectrometer.

The increased resolution of the spin flip laser produces about ten times

35 more lines than were obtained by previous means in the region where the

spectra overlap. The spectrum shown consists of some v transitions, however,

36

the majority of the lines result from the v, mode and the spectrum will

require considerable effort to unravel.

In order to explain this spectrum and obtain some useful parameters,

one would like to make accurate frequency measurements on some of the line

positions. There are several accuracy levels and techniques that can be

used here.

37

One method which has been used at Heriot Watt and could be put to

use here is to use a known calibrant (the NO spectrum in Fig. 11 for

example) along with a low finesse Fabry Perot cavity for interpolation.

The unknown spectrum is compared with the calibrant over identical pump

frequency and magnetic field regions.

A preferable but more difficult technique (and the one we are working

toward here) is to replace the wavelength metrology with frequency metrology

by a incorporating the frequency synthesis technology into the system.

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acoustic detector. Spectrum consists mainly of v. and 2v_ transitions in NH . The spin flip laser was operated in the quasi continuous

(spin saturated) regime with a linewidth the order of 300 MHz; a very

useful mode for survey work. Several different pump lines were used

to obtain spectra in the various 23 cm regions that a 1 Tesla

sweep produces, then the traces were subsequently joined together. A

future improvement will consist of normalizing the absorption signals

by a ratio detection scheme in order to simplify the assignment of

t rans i t ions

.
A block diagram for a scheme currently under investigation is shown in Fig.

17- The spin flip laser is tuned to some resonance feature in the gas

under investigation, and the spin flip laser frequency is measured relative

to the CO pump frequency. The CO pump frequency is known since it is frequency

locked to a CO based reference. This CO laser stabilization has already

been accomplished. The more difficult measurement is the CO - SF frequency

difference. Since the pump electric field and the SFRL field are presumable

polarized orthogonally, the use of the Brewster angle beam splitter should

effectively separate the two signals and permit one to bring the two signals

to the diode independently. The major difficulty to date has been the

relatively low SFRL output power. We have achieved 30 mw near resonance,

but the power drops off toward longer wavelengths. The diodes seem to

operate reliably with about 100 mw of power focused through an F5 lens. We

have succeeded in observing a rectified signal from the SFRL radiation.

The amplitude of this signal was less than 100 uv.

The low power from the SFRL may be partly caused by bounce modes

which approach the InSb surface internally at an angle greater critical angle

-> o and are permanently internally reflected until they come to a corner.

The most promising answer to the problem appears to be that of external cavity

39 operation. " With this operation one expects not only higher output power,

but also a narrower linewidth since bounce modes would be eliminated. An

alternate solution for the CO pump-SFRL frequency difference measurement

might be a. fast solid state detector which requires considerably less power

for operation. To date, even the best of these, however, has only 65 GHz

or so bandwidth. Thus, the much wider bandwidth of the MOM diode makes it

preferable at present, and we are continuing in this direction.

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In summary, we make the following points about the spectrometer:

1. The system has developed to the point where the spin flip laser

operation is routine, and further improvements in performance require

external cavity operation. 2. The present system is useful for survey work

where operation is in the spin saturation mode and tuning is quasi continuous

with a linewidth of about 300 MHz. 3- The system can now be operated in a

- -| mode modulated fashion with a resolution of better than 0.01 cm . k. By

using a ratio detection scheme now on hand, one can measure chemical reaction

rates if one of the reactants has a spectral feature in the 1 900- 1 840 cm

region. 5- An additional effort is required to develop the absolute frequency

measurement capability.

We wish to express our gratitude to Dr. J. R. McMesby of the 0AWM

Headquarters, NBS, Washington for valuable support of the aspects of this

work dealing with environmental problems. We are indebted to Drs. A. Mooradian

and S. R. J. Brueck of the MIT Lincoln Labs and to Prof. S. D. Smith and Drs.

R. B. Dennis, M. J. Colles and others of Heriot-Watt University for valuable

technical discussions on the spin flip laser. Several of our local colleagues

have been helpful, we especially want to thank Drs. K. M. Evenson and J. J.

Jimenez for helpful pointers on the diode, Dr. D. A. Jennings for valuable

comments on various aspects of the project, and Dr. R. J. Mahler for a

critical review of the manuscript. Dr. B. Woodward, who is now at Molectron,

also made some prior contributions to the spin flip laser development.

hi
8. REFERENCES

1- Smith, S. D. , "High resolution infra-red spectroscopy: the spin-flip

laser" invited paper at the Rank Fund Symposium on Very High Resolution Spectroscopy at the Royal Society, London 5

_ 6 Sept. 1974, Heriot

Watt Physics Dept . , Heriot Watt University, Riccarton, Currie, Edinburgh,

Scot land

.

2. Guerra, M. A., Brueck, S. R. J., and Mooradian, A., "Gradient-field

permanent-magnet spin-flip laser," IEEE J. Quantum Electronics, Vol.

QE-9, No. 12, Dec. 1973, pp 1157-1158.

3- Evenson, K. M. , Wells, J. S., Petersen, F. R., Danielson, B. L-, and

Day, G. W. , "Accurate frequencies of molecular transitions used in laser

stabilization: the 3-39 um transitions in CH. and the 9-33 and 10.1

4. urn transitions in CO " Appl. Phys. Lett., Vol. 22, No. 3, Feb. 1973

pp. 192-195.

4. Petersen, F. R. , McDonald, D. G., Cupp, J. D., and Danielson, B. L. "Rotational constants for ''C'^c^ from beats between Lamb-dip stabilized

lasers," Phys. Rev. Lett. Vol.31, pp. 573~ rJ6, 1 973 -

5- Freed, C, and Javan, A., "Standing-wave saturation resonances in the

CO 10.6 um transitions observed in a low-pressure room-temperature

absorber gas," Appl. Phys. Lett., Vol. 17, 1970, pp. 5.3-56.

6. Goldan, P. D., and Goto, K. , "An acoustically resonant system for

detection of low level infra-red absorption in atmospheric pollutants,"

J. Appl. Physics, Vol. 45, No. 10, Oct. 1974, PP- 4350-4355-

7. Brueck, S. R. J., and Mooradian, A., "Near resonance spin-flip scattering

in indium antimonide," Phys. Rev. Lett. Vol. 28, No. 3, Jan. 1972,

pp. 161-165.

8. Patel, C. !(. N., "Tunable spin-flip Raman laser at magnetic field as low as 400 G," Appl. Phys. Lett., Vol. 19, No. 10, Nov. 1971, pp. 400-403-

9. All wood, R. L., Dennis, R. B., Firth, W. J., Smith, S. D., Wherret, B. S. and Wood, R. A., "A tunable spin-flip magneto-Raman infra-red laser,"

The Radio and Electronic Engineer, Vol. 42, No. 5, May 1972, pp. 243"246.

10. Brueck, S. R. J., and Mooradian, A., "Frequency stabilization and finetuning characteristics of a cw InSb spin-flip laser," IEEE J. Quantum

Electronics, Vol. QE-10, No. 9, Sept. 1974, pp. 634-641.

11. MacKenzie, H. A., Smith, S. D., and Dennis, R. B., "Tunning and mode

characteristics at the cw InSb spin-flip Raman laser determined by use of a Fabry-Perot interferometer," to be published.

12. Tennis, R. B., Pidgeon, C. R., Smith, S. D., Wherret, B. S., and Wood,

R. A., "Stimulated spin-flip scattering, a magnetically tunable infra-red

laser I," Proc. Psy. Soc. London, Vol. 331, No. 1585, Dec. 1972, pp. 203"236.

48
13- Brueck, S. R. J., and Mooradian, A., "Efficient, single-mode, cw, tunable

spin-flip Raman laser," Appl . Phys. Lett. Vol. 18, No. G, March 1971,

pp. 229-230.

14. Wherrett, B. S., and Firth, W. J., "Spin saturation and pump depletion

in continuous spin-flip Raman oscillation," IEEE J. Quantum Electron.

Vol. QE-8, Dec. 1972, pp. 865-868.

15. Scott, J. F. , "Spin-flip light scattering and spin-flip lasers,"

Lectures in 1973 Summer School in Crystal Mountain, Washington,

Book, Physics of Quantum Electronics, Vol. II; Laser Applications to

Optics and Spectroscopy, pp. 123 - l80, Ed. S. F. Jacobs, M. Sargent, III,

J. F. Scott and M. 0. Scully (Add i son-Wes 1 ey Pub. Co., Inc., Reading,

(lass. 1975).

16. Colles, M. J., Dennis, Smith, J. W., and Webb, J. S., "A study of

factors affecting the InSb cw spin-flip laser," Cptics Communications

Vol. 10, No. 2, Feb. 197*+, pp. 145-148.

17- Fhelan, R. J., private communication.

18. Mantz, A. W. , Ma i 1 1 ard , J . -E? . , Roh , W. B., and Rao, K. N., "Ground state

molecular constants of C 0," J. Mol . Spectrosc. 5_7, 155 ( 1 975 ) -

19- Loewin, E. G., Maystre, D., McPhedran, R. C, and Wilson, I., "Correction

between efficiency of diffraction gratings and theoretical calculations

over a wide range," Proc. ICO Conf. Opt. Methods in Sc i . and Ind.

Meas., Tokyo, 1974, Japan J. Appl. Phys. 14, 1975, pp. 143-152.

20. Freed, C, "Sealed-off operation of stable CO lasers," Appl. Phys. Lett.

Vol. 18, No. 10, May 1971, pp. 458-461.

21. Petersen, F. R. , et al., "Far infra-red frequency synthesis with stabilized CO lasers: accurate measurements of the water vapor and methyl

alcohof laser frequencies," IEEE J. Quant. Electron. Vol. QE- 11, No. 10,

Oct. 1975, pp. 838-843.

22. Jennings, D. A., Petersen, F. R., and Evenson, K. M., "Extension of

absolute frequency measurements to 1 48 THz : frequencies of the 2.0

and 3-5 um Xe laser," Appl. Phys. Lett. Vol. 26, No. 9, May 1975,

pp. 510-511-

23. Sakuma, E. , and Evenson, K. M., "Characteristics of tungsten-nickel

point contact diodes used as laser harmonic-generator mixers," IEEE

J. Quant. Electron. Vol. QE-10, No. 8, Aug. 1974, pp. 599"603-

24. Hocker, L. 0., Javan, A., Rao, D. Ramachandra, Frenkel, L., and Sullivan,

T. , "Absolute frequency measurement and spectroscopy of gas laser transitions in the far infra-red," Appl. Phys. Lett., vol. 10, No. 5, March

1967, pp. 147-149.

49
25- Wells, J. S., Tvenson, K. M. , Day, G. W. , and Halford, Donald, "Role

of Infra-red frequency synthesis in metrol ogy ," Proc. IEEE, Vol. 60,

No. 5, May 19/2, pp. 621-623-

26. Matarrese, L. M., and Evenson, K. M. , "Improved coupling to infrared

whisker diodes by use of antenna theory," Appl . Phys. Lett., Vol. 17,

No. 1 , July 1970, pp. 8-10.

27- Faris, S. M. , and Gustafson, T. K. , "Harmonic mixing characteristics

of metal -barrier-neta 1 junctions as predicted by electron tunnelling,"

Appl. Phys. Lett. Vol. 25, No. 10, Nov. 1974, pp. 544-547.

28. Simmons, J. G., "Electric tunnel effect between dissimilar electrodes

separated by a thin insulating film," J. Appl. Phys. 34, June 1963,

pp. 2581-1590.

29. James, T. C, and Thiebault, R. J., "Spin-orbit coupling constant of

nitric oxide. Determination from fundamental and satellite band origins,"

J. Chem. Phys. Vol. 41 , 1964, pp. 2806-2813.

30. Howard, C. J., and Evenson, K. M., "Laser magnetic resonance study of

the gas phase reactions of OH with CO, NO, and NO ," J. Chem. Phys.

Vol. 61, No. 5, Sept. 1974, pp. 19^3-1952.

31. Nill, K. W. , Blum, F. A., Calawa, A. R., and Harman, T. C, "Observation

of A-Doubling and Zeeman splitting in the fundamental infrared absorption

band of nitric oxide," Chem. Phys. Lett. Vol. 14, No. 2, May 1972, pp.

234-238.

32. Flicker, H. , private communication.

33- Bischel, 17. K. , Kelly, P. J., and Rhodes, C. K. , "Observation of

doppler-free two-photon absorption in the v bands of CH F," Phys. Rev.

Lett. Vol. 34, No. 6, Feb. 1975, pp. 300-303. 3

34. Rhodes, C. K. , private communication.

35- Garing, J. S., Nielsen, H. H., and Rao, K. N., "The low-frequency

vibration rotation bands of the ammonia molecule," J. Mole. Spectrosc.

Vol. 3, 1959, PP. 496-527.

36. Maki, A. G., private communication.

37. Butcher, R. J., Dennis, R. B., and Smith, S. D., "The tunable spin-flip

Raman laser, II continuous wave molecular spectroscopy," to be

published in the Proc. Royal Society (London 1975)-

38. Mooradian, A., lecture notes at Scottish University Summer School in

Physics, Edinburg, Scotland, Aug. 1 975

39. Scragg, T. , and Smith, S. D., "External cavity operation of the spinflip Raman laser," to be published in proceedings of the Megeve

Conference on High Resolution Spectroscopy, June 1975-

50
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN 670

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

An Infrared Spectrometer Utilizing a Spin Flip Raman Laser,

IR Frequency Synthesis Techniques, and CO Laser Frequency

Standards

5. Publication Date January 1976

6. Performing Organization Code

277.03

7. AUTHOR(S) J. S. V/ells, F. R. Petersen, G. E. Streit, P. D, Goldan and C.—fcL—Sadowski

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2773153

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

Department of Commerce

Washington, D.C. 2023*t

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The central part of_|his spectrometer is a cw spin flip Raman laser which operates

between 1 900 and 1800 cm . The spin flip. laser has so far demonstrated capabilities

of resolving spectra separated by 0.01 cm and shows promise of exceeding this resolution capability. When used with an opto-acoust ?c detector or with a detector and a flow system, it is capable of making a variety of measurements of environmental interes

Many of these environmental applications require only wavelength metrology.

By using C0

2 laser standards as frequency references and incorporating infrared

frequency synthesis techniques, spectroscopy with the spin flip Raman laser can be put

on a frequency metrology basis. A CO laser has been used with a metal -oxide-metal

diode to synthesize a known frequency reference which was used to stabilize the CO

pump laser. The MOM diode also has the potential for measuring the frequency

difference between the SFRL and the CO pump laser, a step which will complete absolute

frequency measurements with the SFRL. The progress toward achieving this goal, the

potential capabilities of this unique spectrometer, and some future applications are

discussed.

t.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Air quality measurements; improved resolution spectroscopy; infrared frequency

measurements; infrared spectrometer

spin flip laser; tunable IR laser.

18. AVAILABILITY [£ Unlimited

1 For Official Distribution. Do Not Release to NTIS

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